During the manufacturing of devices by optical lithography such as semiconductor devices, e.g., integrated circuits on a substrate, areas must be selected on this substrate to allow selective execution of processes thereupon such that the features of the integrated circuits can be defined. Examples of such processes are: introduction in these selected areas of controlled amounts of species to modify the electrical characteristics of the exposed substrate, the removal of material from the substrate in the selected areas by dry or wet etch processes to define a pattern in the substrate, etc. Lithographic processes are used to create these selected areas. A lithographic process comprises several steps.
Firstly, a uniform layer of a photosensitive resin, also known as resist is provided over the substrate. Such resist is commercially available from companies such as Arch (USA), SUMITOMO (Japan), Clariant (Switserland) This resist layer is then selectively exposed to a form of radiation, which modifies the chemical properties of the irradiated portions thereof.
Depending on the wavelength of the radiation used, one distinguishes between optical lithography employing wavelengths above 10 nm, such as 248 nm or 157 nm optical lithography and extreme ultra-violet lithography (“EUV”), on the one hand, and non-optical lithography to which techniques such as electron-beam lithography (“e-beam”), X-ray lithography or ion-beam lithography belong, on the other hand. Typically, a photomask, also known as a mask or reticle, is used to selectively expose the resist layer to this radiation. Such a mask comprises a carrier, formed in a material transparent to the radiation, upon which a pattern is formed in a non-transparent material that, where present, blocks the propagation of light. Hence, this mask pattern will determine where radiation will impinge upon the substrate. Typically a thin chromium layer is used as light-blocking material while fused silica can be used as transparent material, although other light-blocking materials and transparent materials can be used as well.
The mask pattern itself is the material representation of the two-dimensional layout of one layer of the integrated circuit. Such a two-dimensional layout is often referred to as a design. The initial design can be corrected and optionally comprise additional features or size adjustment, which would correct for deficiencies of the lithographic process. By original design is meant that the design as it is being put on the mask. By original mask is meant the mask containing the design, as it should be ideally formed in the resist. In case of positive tone resist, the irradiated portions become more soluble upon irradiation and these exposed portions can be removed selective to the non-exposed regions thereby forming a positive image of the mask pattern. If the non-irradiated regions become dissolvable, i.e., in case of negative tone resist, a negative image of the mask pattern is obtained. The amount of energy necessary to modify the solubility properties of the resist is a characteristic of the resist. This energy threshold can be tuned by selecting the appropriate resist composition.
After the exposure step, the soluble portions of the resist layer are removed and corresponding selected areas are formed in the resist layer through which the underlying substrate is exposed. As a result a pattern of resist is created on the substrate serving as a mask for the processes defining the features of, e.g., the integrated circuit.
The downscaling of semiconductor technology reduces the dimensions of the features of the integrated circuit, e.g., lines, holes, spacing between lines and holes. As these dimensions shrink, it becomes more difficult to define such small features. As a correlation exists between the wavelength of the radiation and the minimal dimension of the selected area—the smaller the wavelength, the smaller the minimal dimension obtainable—and one would use non-optical lithography to obtain dimensions of a nanometer scale. However, these non-optical techniques are very expensive, time-consuming or still require a considerable amount of research and development efforts before becoming production-worthy. Therefore there is a tendency to shift the limits of the well-known optical lithography using R(esolution)-E(nhancement)-T(echniques). For example, U.S. Pat. No. 6,686,102 discusses the problem of defining ever-smaller features by optical lithography and discloses the use of a double-exposure phase shift lithography process.
However, a limit is imposed to the improvement in resolution obtainable by such enhancement techniques as optical lithography suffers from the so-called “stray light” or “flare” effects. “Flare” is an optical effect causing irradiation of the non-selected areas of the resist layer, as light will spread out or will be scattered within the lithographic exposure tool causing the dimensions of the features printed in the resist to change from their desired values. Kafai Li et al. discuss in “Scattered light: the increasing problem for 193 nm exposure tools and beyond”, SPIE proceedings vol. 4346 (2001), the origin of stray light and the fact that stray light becomes more and more critical when maximizing the performance of optical lithography systems, especially when using smaller wavelengths.
FIG. 1a to FIG. 1f illustrate the influence of stray light on the dimensions of various patterns to be formed in a layer of resist during optical lithography. FIG. 1a shows a cross-sectional view of a mask 100 comprising a light-blocking layer 102, such as, e.g., a chromium layer, formed upon a transparent carrier 104. The light-blocking layer 102 shown by way of example in FIG. 1a contains a pattern of a small isolated opening 106, a grating 108, i.e., for example a sequence of 3 openings or of 2 lines, an isolated opening 110 and an isolated line 112. FIG. 1b shows, as a function of the position x along the mask 100, for each position whether radiation is being transmitted, represented by an intensity I equal to “1”, or is being blocked, represented by an intensity I equal to “0”. Such pattern data can be considered as a digital representation of the pattern present in a layer. In the ideal case, the spatial distribution of the radiation leaving each opening within the light-blocking layer 102 can be represented by a point-spread function having a box profile, or mathematically by a Kronecker delta-function. However, in reality the transmitted light will spread out as illustrated for the isolated opening by FIG. 1c. Instead of a box-like point-spread function, a decaying intensity profile 120 of the stray light is obtained which can stretch out over a distance ranging from a few micrometers up to a few hundred micrometers depending on the lithographic system used. Such a decaying point-spread-function is characterised by its decay constant λ.
After being transmitted by the openings in the mask 100, the transmitted light will impinge upon a substrate 150 comprising a resist or photosensitive layer 140 provided on a layer to be patterned 142 (shown in FIG. 1e). A substrate 150 may include any suitable carrier, such as, e.g., a semiconductor substrate, a glass substrate, a metal substrate, a plastic substrate, etc. Alternatively, a substrate also may include a layer or layer structure on a carrier, such as a device or a circuit on a carrier. The layer 142 to be patterned can also be a part of the carrier material. For the ease of description, it is assumed that the substrate 150, and more specifically the layer 142 to be patterned, is already covered with a photosensitive layer 140. In case of positive tone resist, the pattern of the mask 100 should, in the ideal case, i.e., without flare, be exactly transferred into the resist when the irradiated portions of the resist are removed. In the ideal case the pattern data of the resist, i.e., the information where resist is removed or remains, should match the pattern data of the mask pattern, i.e., where the light blocking layer 102 is present or absent, as through each of these openings light is transmitted in the form of parallel rays towards the resist 140. But, as the transmitted light spreads out, also portions not aligned to the openings in the mask will be irradiated to an extent determined by the decaying light intensity profile of the transmitted light.
FIG. 1d shows the logarithm of the energy absorbed in the resist 140 as function of the position x along the layer 142, while FIG. 1e shows a cross-sectional view of the substrate 150, comprising the positive tone resist 140 overlying layer 142. This view is made after exposure by the pattern of FIG. 1a taking into account the threshold of the resist depicted by the dotted line “a” in FIG. 1d. The threshold of the resist refers to the dose of energy necessary to modify the solubility properties of the resist. In case of the area 156 corresponding with the small isolated opening 106 and the area 160 corresponding with the isolated opening 110, the radiation is spread out over a larger area of the resist 140 thereby resulting in a widening of the resist area which is exposed to radiation. This is indicated by the arrows b for the area 160 corresponding with the isolated opening 110 whereby the dashed lines within this opening indicate its dimension without taking the flare effect into account. In relative terms this effect is more pronounced for the smallest isolated opening 106. In case of the grating 108 or the isolated line 112, the absorbed energy distributions overlap in the corresponding areas 158 respectively 162. The non-exposed region corresponding to isolated line 112 has nearly disappeared as it is overexposed by the flare radiation coming from both large openings at either side of the isolated line 112 in the mask 100. Instead of obtaining a step-like energy profile I in the resist 140, the stray radiation causes this energy profile I to be spread out thereby introducing a background energy dose in the resist 140.
FIG. 1f shows the resulting pattern data in the resist in case positive tone resist is used. A “1” level corresponds to a position which was not exposed and a “0” corresponds to a position where light was absorbed and hence the positive tone resist became soluble. The openings 156, 160 in the resist became larger, the openings of the grating 158 were merged together in one large opening and the single line 112, separating two large openings, has disappeared yielding one large opening 162 in the resist. This deviation in dimension of the resist features (see FIG. 1e) compared to the corresponding features of the mask features (see FIG. 1a) depends on the rate at which transmitted light decays, i.e., the spread of the transmitted light, and on the threshold of the resist, i.e., the amount of light energy required to render the irradiated regions soluble. Mathematically spoken this effect is identical to the convolution of the pattern data of the mask, shown in FIG. 1b, with the point-spread function of the transmitted light, illustrated in FIG. 1c. So flare will result in a deviation of the desired dimensions of the features of the integrated circuit and this stray light will limit the performance of the optical lithography tool.
One way to reduce the impact of stray light on dimension is to reduce the amount of stray light in the optical lithographic tool itself, e.g., by reducing surface roughness or by surface coating. This however requires considerable redesign of the tool.
Another way to solve the dimension or feature size variation induced by flare is to modify the original design and to change the lateral dimensions of the pattern taking into account the induced variation. In the example illustrated by FIG. 1a to FIG. 1f this would imply that the lateral dimensions, i.e., the diameter in case of a circular opening, of the two outer openings would be reduced to an extent determined by the spread of the transmitted light as given by its point spread function. This bias of the lateral dimensions would hence compensate upfront for the later induced variation. However modifying the original design is time consuming and calculation intensive requiring a considerable amount of computer effort. As such small changes in the original feature size necessitate a higher manufacturing accuracy, the manufacturing cost of the mask will increase. Furthermore, although only one exposure step is used, a lithographic process with a higher resolution might be required to allow printing of such slightly modified features and hence the processing cost might increase.
Published U.S. application 2004/0010768 discloses another method for reducing the influence of the spread of the transmitted light on the feature size. The aim of this method is to generate an identical pattern surrounding those features requiring a precise definition of their dimensions, i.e., the “target features”, thereby resulting in a reproducible and similar exposure environment for each such target feature. Whereas in the ideal case only one exposure step would be needed using the original mask to form the pattern in the resist, the disclosed method comprises two exposure steps. Each exposure step requires the use of a revised mask, each of these revised masks being a different modification of the original mask. This approach requires two masks and for each mask its suffers from the same disadvantages as the previous method.